In an electrostatic recording process performed by a copy machine, printer or the like, conventionally, printing is carried out by a method comprising the steps of: first, uniformly charging the surface of a photoconductor (latent image-retaining member); projecting from an optical system an image onto this photoconductor; removing the charge from the part exposed to light to form an electrostatic latent image; subsequently providing a toner to the thus formed electrostatic latent image to form a toner image by electrostatic adsorption of the toner; and transferring the thus formed toner image onto a recording medium such as paper, OHP or photographic paper.
Also in a color printer and color copy machine, printing is performed basically in accordance with the aforementioned process; however, in the case of color printing, since color tones are reproduced by using four toners of magenta, yellow, cyan and black, it is necessary to carry out a step of overlaying these toners at a prescribed ratio to obtain desired color tones. Several techniques have been proposed for performing this step.
The first example of such technique is image-on-image development method. In this method, for visualization of an electrostatic latent image formed on a photoconductor by providing toners, an image is, in the same manner as in black-and-white printing, developed by sequentially overlaying the aforementioned four toners of magenta, yellow, cyan and black, thereby forming a color toner image on the photoconductor. This method enables a relatively compact apparatus configuration; however, it has a problem in that a high quality image cannot be obtained since gradation control is extremely difficult.
The second example of the aforementioned proposed technique is tandem method. In this method, a color image is reproduced by the steps of: developing latent images on each of four photoconductor drums using magenta toner, yellow toner, cyan toner and black toner to form each toner image by magenta, yellow, cyan and black; and sequentially transferring the toner images thus formed on the photoconductor drums, which are configured in a line, onto a recording medium such as paper to overlay the toner images on a recording medium. This method provides a good quality image; however, the apparatus is large and expensive since the four photoconductor drums, each of which has a charging mechanism and a developing mechanism, have to be configured in a line.
Illustrated in FIG. 2 is an example of the constitution of a printing unit of an image-forming apparatus used in such tandem method. The printing unit has four printing units aligned sequentially corresponding to each toner of yellow (Y), magenta (M), cyan (C) and black (B), each printing unit of which is constituted by a photoconductor drum 1, a charging roller 2, a developing roller 3, a developing blade 4, a toner feeding roller 5 and a cleaning blade 6. The toners are sequentially transferred onto a sheet as it is carried by a transfer conveyer belt 10, which is circularly driven by driving rollers (driving member) 9, thereby forming a color image. Charging of the transfer conveyer belt and charge-removal therefrom are carried out by a charging roller 7 and a charge-removing roller 8, respectively. In addition, an adsorption roller (not shown) is used to charge the sheet in order to allow the sheet to adsorb onto the belt. By having such the above constitution, generation of ozone can be suppressed. The adsorption roller not only transfers the sheet from a sheet feeding path to the transfer conveyer belt, but also performs electrostatic adsorption of the sheet onto the transfer conveyer belt. Further, separation of the sheet therefrom after image transfer can be carried out solely by self stripping since the adsorption force between the sheet and the transfer conveyer belt becomes low by lowering the transfer voltage.
The material of the transfer conveyer belt 10 may be resistive or dielectric; however, each material type has its advantages and disadvantages. Since a resistive belt retains charges for only a short duration, in cases where such resistive belt is employed for the tandem-type transfer, there is only a limited amount of charge injection during the transfer and the increase in the voltage is relatively small even when four colors are consecutively transferred. Furthermore, even in cases where the resistive belt is repeatedly employed to consecutively transfer sheets, it is not required to electrically reset the belt since charges thereon should have been already released by the time of transferring the next sheet. However, such resistive belt has disadvantages in that the transfer efficiency is affected by environmental variations as the resistance value varies depending on the environmental variations, and that it is likely to be affected by the thickness and width of the printing sheet.
In contrast, a dielectric belt does not spontaneously release injected charges; therefore, injection and release of charges have to be controlled electronically. However, since the charges are stably retained by the belt, sheet adsorption is assured and sheet transfer is performed at a high accuracy. In addition, as the dielectric constant is less dependent on the temperature and humidity, the transfer process is relatively stable against environmental variations as well. A disadvantage of such dielectric belt is that charges are accumulated from repeated transfers, thereby increasing the transfer voltage.
The third example of the aforementioned proposed technique is transfer drum method in which a color image is reproduced by rotating a transfer drum, which is lapped with a recording medium such as paper, four times, in each of which rotation magenta, yellow, cyan and black toners on a photoconductor are sequentially transferred onto the recording medium. This method yields an image having a relatively high quality; however, in cases where the recording medium is a thick paper such as a postcard, since lapping of such recording medium around the transfer drum is difficult, this method has a problem in that the type of the recording medium is limited.
As an alternative method to the aforementioned image-on-image development method, tandem method and transfer drum method, intermediate transfer method, which yields a good quality image without particularly increasing the size of the apparatus and limiting the type of the recording medium, has been proposed.
That is, in this intermediate transfer method, an intermediate transfer member comprising a drum and a belt which transfer and temporarily retain a toner image formed on a photoconductor is provided, and around this intermediate transfer member, four photoconductors each having a toner image with magenta, a toner image with yellow, a toner image with cyan and toner image with black are arranged. The toner images of four colors are sequentially transferred onto the intermediate transfer member from the photoconductors, thereby forming a color image on this intermediate transfer member, which color image is then transferred onto a recording medium such as paper. Therefore, since the gradation is adjusted by overlaying the toner images of four colors, a high quality image can be obtained. At the same time, the apparatus does not have to be particularly scaled up since there is no need to lineally arrange the photoconductors as in the case of tandem method, and the type of recording medium is not restricted as the recording medium does not have to be lapped around the drum.
As an apparatus to perform color-image formation by such intermediate transfer method, FIG. 3 illustrates an image forming apparatus which comprises an intermediate transfer member in the form of an endless belt.
In FIG. 3, indicated as 11 is a drum photoconductor which rotates in the direction of the arrow. This photoconductor 11 is charged by a primary charging unit 12 and an image exposure unit 13 subsequently removes the charge from the part exposed to light, forming an electrostatic latent image corresponding to a first color component onto the photoconductor 11. By a developing unit 41, the thus formed electrostatic latent image is then developed with the first color, magenta toner (M), to form a toner image of the first color, magenta, onto the photoconductor 11. Subsequently, this toner image is transferred onto an intermediate transfer member 20, which is being circularly rotated in contact with the photoconductor 11 by a driving roller (driving member) 30. In this step, the image transfer from the photoconductor 11 onto the intermediate transfer member 20 is carried out at the nip portion between the photoconductor 11 and the intermediate transfer member 20 by primary transfer bias applied from a power source 61 to the intermediate transfer member 20. After the transfer of the toner image of the first color, magenta, onto this intermediate transfer member 20, the surface of the aforementioned photoconductor 11 is cleaned by a cleaning unit 14, thereby completing the first round of the image development and transfer operation by the photoconductor 11. In each of the subsequent three rotations of the photoconductor, by sequentially using developing units 42 to 44, a toner image of second color, cyan; a toner image of third color, yellow; and a toner image of fourth color, black, are sequentially formed onto the photoconductor 11 and superimposed onto the intermediate transfer member 20. Consequently, a composite color toner image corresponding to the desired color image is formed onto the intermediate transfer member 20. In the apparatus shown in FIG. 3, after each rotation of the photoconductor 11, the developing units 41 to 44 are sequentially placed into the position to perform sequential development by the magenta toner (M), cyano toner (C), yellow toner (Y), and black toner (B).
In the next step, the intermediate transfer member 20 onto which the aforementioned composite color toner image has been formed comes in contact with a transfer roller 25, and to the nip portion thereof, a recording medium 26 such as paper is fed from a paper feeding cassette 19. Simultaneously, secondary transfer bias is applied from a power source 29 to the transfer roller 25 and the composite color toner image is transferred and heat-fixed onto the recording medium 26 from the intermediate transfer member 20, thereby forming a final image. After the transfer of the composite color toner image onto the recording medium 26, residual toners on the surface of the intermediate transfer member 20 are removed by a cleaning unit 35 to return the intermediate transfer member 20 to the initial condition for the next image formation process.
There is also an intermediate transfer method combined with the tandem method. FIG. 4 illustrates an image forming apparatus of intermediate transfer method in which color images are formed using an intermediate transfer member in the form of an endless belt.
In the illustrated apparatus, a first developing unit 54a to a fourth developing unit 54d, which develop electrostatic latent images on photoconductor drums 52a to 52d with yellow, magenta, cyan and black, respectively, are sequentially arranged along an intermediate transfer member 50. This intermediate transfer member 50 is circularly driven in the direction of the arrow, and thereonto, toner images of four colors that have been formed on each of the photoconductor drums 52a to 52d of developing units 54a to 54d are sequentially transferred, thereby forming a color toner image onto this intermediate transfer member 50. The thus formed color toner image is then transferred onto a recording medium 53, such as paper, to be printed out. In any of the aforementioned apparatuses, the sequence of the toners used in the image development is not particularly restricted and can be arbitrarily selected.
In FIG. 4, the symbol 55 represents a driving roller or tension roller for circularly driving the intermediate transfer member 50, and the symbols 56 and 57 represent a recording medium feeding roller and recording medium feeding device, respectively, while the symbol 58 represents a fixing device which fixes an image on a recording medium by heating or the like. Further, the symbol 59 represents a power source unit (voltage applying means) which applies a voltage to the intermediate transfer member 50. This power source device 59 can reverse the positive and negative of the voltage to be applied between when transferring toner images to the aforementioned intermediate transfer member 50 from the photoconductor drums 52a to 52d and when transferring the thus formed color toner image from the intermediate transfer member 50 to the recording medium 53.
Conventionally, for the electroconductive endless belt used as the transfer conveyer belt 10 and intermediate transfer members 20 and 50 and the like, semi-conductive resin film belts or rubber belts having a fiber-reinforced member body have been mainly used. Among such resin film belts, for example, those of which polycarbonate (PC) is mixed with carbon black, those comprising a polyalkylene terephthalate as the principal resin, and those comprising a thermoplastic polyimide as the principal resin, are known.
In addition, for example, Patent Document 1 discloses an unstretched seamless belt comprising a thermoplastic polyalkylene terephthalate and carbon black. Further, Patent Document 2 discloses a seamless belt which comprises a resin composition obtained by adding a functional component to a resin component containing a polyalkylene terephthalate (PAT) in combination with a thermoplastic elastomer having a melting point of not lower than 100° C. and not higher than that of the PAT and a compatibility parameter difference from that of the PAT by less than 2.0. Still further, Patent Document 3 discloses an electroconductive endless belt which contains a thermoplastic polyalkylene naphthalate resin, a base material of other thermoplastic resin having an ester bond, and an electroconductive material, which electroconductive endless belt has a prescribed MFR value of the thermoplastic resin having an ester bond at 270° C. and a prescribed weight ratio between the thermoplastic polyalkylene naphthalate resin and the thermoplastic resin having an ester bond.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 06-149081 (the claims and the like)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 08-099374 (the claims and the like)    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-266760 (the claims and the like)